Light emitting diodes are gaining popularity to be used as light sources for various applications. Since the demonstration of high quality Gallium Nitride crystal growth, high efficiency Indium Gallium Nitride quantum well blue LED chips emitting at around 450 nm (blue) wavelength have been intensely developed worldwide. White LED chips obtained from phosphor converted blue LEDs are now used in general lighting and backlighting applications. In particular, LED bulbs are being developed as replacement lamps for incandescent bulbs, halogen bulbs, cold cathode fluorescent lamps (CCFL) and compact fluorescent lamps (CFL) for general lighting applications. However, LEDs are surface mounted devices and light emission has a Lambertian profile with 120° beam angle. Hence the beam angle of the light distribution for LED bulbs commercially available to date is typically 120°-140°. In comparison conventional incandescent or CFL bulbs produce light distribution beam angles larger than 270°. While the LED narrow beam angle property makes LED bulbs ideal for down-lighting applications (mounted on ceiling), it is not ideal for vertical-lighting applications, whereby light is not distributed to the bottom or sides well enough. This results in uneven light distribution and shadowing effect. Conventional methods of using diffusive coating on an LED dome casing to increase scattering and subsequently the beam angle typically results in light loss of 20-30%, resulting in reduced emission efficiency. The angular distribution of the light output for these conventional methods is typically <150° and with different profiles to a conventional CFL bulb.
FIG. 1A shows the Lambertian profile for an LED with a beam angle of 120° on a polar diagram. In FIG. 1B, the angular light distribution of a conventional CFL bulb is shown. Observe that the angular distribution is up to >300° for a CFL bulb, and light intensity distribution is fairly uniform.
FIG. 2A is an example of a commercially available LED bulb using a dome diffuser to improve beam angle and reduce glare. The LED 1 is mounted on a metal heat-sink chassis 2 (typically with fins to increase surface area for redistributing heat) for heat dissipation. In particular for high power LED bulbs, the size of the metal heat-sink chassis 2 is usually increased to dissipate heat more efficiently. Increasing the metal heat-sink area will increase the total LED bulb size and weight, making the light bulb design poorer and less appealing to consumers. The LED bulb casing 3 is usually dome shaped and coated with a diffusive coating 4 to increase beam angle and reduce glare. FIG. 2B shows the polar diagram of an LED bulb with the features described in FIG. 2A. Although the beam angle is increased from FIG. 1A, the uniformity of light intensity distribution is poor, and the angular distribution very different to a CFL bulb. Beam angle is also reduced from this structure, since the large metal heat-sink prevents light from being directed downwards. The diffusive coating 4 is a source of light loss, reducing the LED bulb efficiency.
To overcome some of these issues, LED light bulbs with various configurations have been developed. These are described in the following paragraphs.
FIG. 3 is the schematic diagram of a method to increase LED bulb light distribution isotropy as described in U.S. Pat. No. 6,450,661 B1 (Okumura et al., September 2002). This LED bulb 100 uses a series of LEDs mounted in a disc-like globe distribution to achieve a wide beam angle light distribution, since half the LEDs and therefore their light emission are directed upwards and the other half downwards. In this configuration, heat-sinking of LEDs is not optimal, since the LEDs are not directly mounted onto a heat-sink. The complexity of mounting LEDs in this manner will also increase manufacturing costs. In particular, heat sinking is critical for high power LED bulbs whereby LED efficacy and lifetime are strongly dependent on its junction temperature. Therefore the method described in this citation will be limited to low power LEDs.
FIG. 4 is a schematic diagram of an LED bulb with an increased light beam angle as described in US2008/0062703A1. The LEDs are mounted in a conformal/three-dimensional-like manner, and light is emitted sideways creating a wider beam angle. Good heat-sinking is difficult to achieve in this method, since the LEDs are only in direct contact with a small area of the heat-sink. This method will also be difficult to implement for an LED bulb using a single LED instead of multiple LEDs.
FIG. 5 is a schematic diagram of a method to produce sideways emitting LEDs as described in U.S. Pat. No. 6,598,998 B2 (West et al., November 2002). A reflective mirror or lens 128 is placed above the LED 130 and light is redistributed sideways and escapes through a saw tooth portion. This method can produce a wider beam angle LED device, but will create a dark spot directly above it, which results in shadowing effects that are not desirable.
FIG. 6 is an LED bulb 140 as disclosed in US2003/0021113 A1 (Begemann, January 2003). In this configuration, the LEDs are mounted to a regular polyhedron 142 with at least 4 faces at an elevated height above the LED bulb screw base, to avoid light distributed downwards being shielded by the heat-sink. This method is not suitable for single module LED bulbs and manufacturing costs will also be higher compared to bulbs with LEDs mounted directly on the heat-sink chassis. This is a consequence of the complexity of the mounting geometry of the proposed configuration.
U.S. Pat. No. 7,329,029 B2 (Chaves et al., October 2005) describes the use of a transfer section 150 and an ejector 152 for distributing the radiant emission from the LED 154 as shown in FIG. 7. In this configuration the LED 154 is placed adjacent the transfer section 150 and the ejector 152 is designed in such a way to produce a light angular distribution higher than 120°. Although this invention describes an enhanced light angular distribution, the optical device used is very complex and would be difficult to manufacture cheaply.
Thus there is a need in the art for LED devices with light distribution superior to 120° which maintain good heat sinking in order to provide high efficiency.
An object of the present invention is to provide an LED device with wide beam angle, good heat dissipation properties, and minimal light loss when creating wide beam angle. It is also an object to provide a method for low cost volume manufacturing of such an LED device. This is important since cost is a key issue for LED device to be competitive against conventional incandescent and CFL lamps.